INTRODUCTION: Sequoia’s Acoustic Backscatter Sensor, LISST-ABS is designed to measure suspended sediment concentration at a point. Unlike ADCP’s which may record similar backscatter strength at multiple points along the acoustic beams (‘ranges’), the LISST-ABS only reports backscatter signal strength at a single point about 5 cm from the transducer. The instrument presents the strength of this backscatter to the underwater connector as (i) analog voltage, (ii) RS-232 digital data, and (iii) SDI-12. The instrument does not include a battery or data logger.
The LISST-ABS employs a high frequency of 8MHz. This choice is made so that the calibration of backscatter signal strength with sediment concentration remains nearly constant over a wide particle size range. The LISST-ABS calibration changes only about ~±30% over its mean value over size 40-500 microns. This is one of the key advantages of LISST-ABS over turbidity sensors, whose calibration changes ~ ±600% over its mean over this same size range. The explanation of this key difference between LISST-ABS and turbidity sensors is described further in this Application Note. All LISST-ABS instruments are factory calibrated with equal sensitivity so that they may be interchanged.
The LISST-ABS is capable of measuring high sediment concentrations due to a key second feature: the backscatter strength is measured from two adjacent range cells, 5 cm apart; i.e. the first range cell is at 5 cm from the instrument, and the second range cell is at 10 cm. The backscatter signal from the second range cell is weaker due to two reasons: the spreading of the acoustic beam reduces signal strength, and second, suspended sediment causes attenuation of sound. The LISST-ABS instrument firmware corrects for both factors, thus providing an Attenuation Corrected Backscatter (ACB). The attenuation correction feature permits use of LISST-ABS to very high concentrations.
The LISST-ABS is designed to work with an 11-18 VDC external power source and data storage device.
DATA Conversion: The ACB output of the LISST-ABS is not directly proportional to sediment concentration. The conversion of the ABS counts of digital data (units: counts) into sediment concentration is done by a simple formula:
where C is the concentration in mg/L, ACB is the digital output of the LISST-ABS, and Nref is a typical value for a particular sediment depending on sediment size and mixture. For 5-10 micron Arizona dust particles used in factory calibration, Nref = 1000, so that at ACB=1000, the concentration is 1mg/L. This factor is the only factor that is changed on recalibration for a different sediment grain size.
Software provided with the instrument enables a one-button recalibration by a user. Please see the User’s Manual for details.